Purpose: An integrated system for online MRIgRT comprising an MR scanner-on-rails proximal to a Varian TrueBeam linac is currently under installation at our institution. This study presents the development of a procedure to commission this platform along with the synthesis, testing, and validation of several unique tools/methods central to this challenging frontier.

Methods: The commissioning procedure is organized into three phases: 1) the linac and a modified patient couch are commissioned in the absence of magnetic fields; 2) the MR magnet is energized, followed by experimental field mapping and verification of linac/couch performance proximal to the magnet; 3) MR imaging is evaluated and integrated guidance functionalities are tested. Novel methods/technologies embedded in these steps include: magnetic field finite element simulations to confirm magnetic decoupling of the MR scanner, linac, and couch; development of a novel harmonic distortion analysis/shim evaluation technique and associated phantom design incorporating comprehensive imaging QA; simulation of patient-induced susceptibility distortions and pulse sequence optimization; development of a phantom/method to evaluate the accuracy/stability of MR scanner/couch travel.

Results: The finite element simulations of the MRIgRT platform indicated acceptable magnetic forces on critical couch/linac components, undisturbed x-ray beam production, and acceptable field uniformity within the MRI active volume (via active shimming). Validation of the harmonic distortion analysis demonstrated that complete volumetric mapping of the geometric distortion could be performed with an error bound by the imaging voxel size. Susceptibility-induced distortions were simulated for various anatomical sites and can be reduced to within 1-2 mm by increasing the frequency encoding gradient strength with acceptable SNR losses. Lastly, the accuracy of translation and rotation measurements was established for evaluating MR scanner/couch travel.

Conclusion: A comprehensive procedure has been established for the commissioning of an integrated MRIgRT platform, and various key tools/methods for assessing system performance have been developed and tested.

Funding Support, Disclosures, and Conflict of Interest: J. Winter and L. Petropoulos are employees of IMRIS. J. Marle and M. Sweitzer are employees of Varian Medical Systems. IMRIS and Varian are companies co-developing the MRgRT system. Funding for the MRgRT project at Princess Margaret Hospital was provided by the Canadian Foundation for Innovation.